The invention relates to a method for transmitting data by radio and to a transmitter for transmitting data by radio via a number of antennas.
In radio communication systems data (for example speech, picture information, video information, SMS (Short Message Service) or other data) is transmitted with the aid of electromagnetic waves via a radio interface between sending and receiving stations. The electromagnetic waves in such systems are radiated using carrier frequencies which lie within the frequency range provided for the relevant system. A radio communication system in this case includes subscriber stations, e.g. mobile stations, base stations, e.g. node Bs or other radio access devices, as well as further network-side devices where required.
To guarantee that data is transmitted as efficiently as possible the overall frequency band available is divided up into a number of subcarriers (multi-carrier method). The basic idea underlying multi-carrier systems, also referred to as OFDM (Orthogonal Frequency Division Multiplexer) systems, is to translate the initial situation of the transmission of a broadband signal into the transmission of a quantify of narrowband orthogonal signals.
With OFDM time pulse shapes which are almost rectangular are used for the subcarriers. The frequency spacing of the subcarriers is selected such that in the frequency space for that frequency at which the signal of a subcarrier is evaluated, the signals of the other subcarriers exhibit a zero crossing. The subcarriers are thus orthogonal to each other. A spectral overlapping of the subcarriers and as a result a high packing density of the subcarriers is allowed, since the orthogonality ensures that the individual subcarriers can be distinguished. A better spectral efficiency than with simple FDM (Frequency Division Multiplexing) is thus achieved.
Because of the dispersion behavior of radio channels it is advantageous, in OFDM systems, to introduce a diversity, into the space area for example, in order to increase the reliability of data transmission. To do this a plurality of antennas are used on the transmit side which each transmit the same information. On receipt the signals subject to different fluctuations are combined. Examples of such methods are the CDD (Cyclic Delay Diversity) method presented in                A. Damman, S. Kaiser: Low Complex Standard Conformable Antenna Diversity Techniques for OFDM Systems and its Application to the DVB-T System, Proceedings of the 4th International ITG Conference on Source and Channel Coding, Berlin 2002,in which virtual echoes of the signal sent on an antenna are created by the different antennas. However in        A. Huebner, M. Bossert, F. Schuehlein, H. Haas, E. Costa: On Cyclic Delay Diversity in OFDM Based Transmission Schemes, Proceedings of the 7th International OFDM Workshop, Hamburg 2002it is shown that this method can only be viably used in combination with channel encoding, such as convolutional encoding, to reduce transmission errors.        
Furthermore, for OFDM transmission with a plurality of antennas, the Alamout method in accordance with                S. M. Alamouti: A Simple Transmit Diversity Technique for wireless Communications, IEEE Journal on Selected Areas in Communications, vol. 16, no. 8, P. 1451-1458, October 1998can be used, in which two consecutive symbols are processed on the transmit side such that two transmit antennas transmit orthogonal signals. A disadvantage of the Alamouti method has proved to be the need to use an expensive, modified demodulator on the receive side, and that a plurality of antennas cannot be used for the transmission without resulting in restrictions as regards the choice of the modulation method.        
Furthermore the “Permutation Transmitter Diversity” method can be used for OFDM transmission with a plurality of antennas, said method being presented for example in Li, Y: “Transmitter Diversity for OFDM Systems and Its Impact on High-Rate Data Wireless Networks”, IEEE Journal on Selected Areas in Communications, Vol. 17, No. 7, July 1999. In this case the signals of an antenna are permutated before of an inverse Fourier transformation in relation to the signals of another antenna.